OFJ Field Journal from Stephen Licata - 10/26/95 (2nd)

GETTING GALILEO DATA INTO YOUR HOT LITTLE HANDS

The Galileo Flight Team spends many months planning complicated science
and engineering activities, but sometimes the most nervous time is spent
waiting for the results to appear on the computer screens at their desks.
As part of my job, every once in a while I call a meeting of the Galileo
Data Flow Working Group to make sure this process runs smoothly at critical
points in the mission. I'd like to share with you one of our recent assignments
- planning the release the Probe from the main spacecraft so that it could
begin its descent into Jupiter's atmosphere.

First, let's look at how the spacecraft data gets to Earth. On Galileo,
the Command and Data Subsystem gathers information on all parts of the
spacecraft (cameras, science instruments, even the heaters and gyroscopes)
and creates a "status report". This report is sent to earth through the
low-gain antenna at a radio frequency around 2300 on your FM dial (Don't
bother trying! The signal is put into a secret code to protect it from
static and interference in space and the Earth's atmosphere). On the ground,
these radio signals are captured at the Deep Space Network by huge tracking
antennas the size of a football field and converted into the computer
language of 1's and 0's. The tracking station computers tell us at what
time the signal was received and then send it by a special network of
wires and radio relay antennas to JPL.

Here at JPL the Data Systems Operations Team converts the data into
a structure that can be understood by JPL's computers and passes the data
on to a set of computers and computer programs called the Multi-mission
Ground Data System (MGDS). The MGDS separates the data into categories
of "spacecraft science data", "spacecraft engineering data", and information
about the tracking station (for example, how strong the radio signal was
when it was received at Earth).

Immediately some of these data go to the Mission Support Area where
the mission controller, a person called the "Ace", looks at the data and
promptly calls the tracking station if there is a problem with the data
stream.

At the same time all the data related to the spacecraft goes to another
part of the MGDS called the Telemetry Delivery Subsystem which is like
a library where people with special permission (like the scientists) can
go to grab the data that they want. If the data they want are parts of
a camera picture, these data first have to go to yet another area called
the multi-mission Image Processing System before they can be looked at
by the scientists and others.

Currently the time required for the data to go from the Deep Space network
to a scientist's computer screen varies from 10 minutes to an hour (for
the pictures). Early next year, we will be changing the computer software
on the Galileo spacecraft such that this time delay may stretch to two
hours but we will be able to get many more pictures each day from the
spacecraft, so scientists are willing to be a little more patient!

In planning the release of the Galileo Probe last summer, we had a few
special problems. First of all, since the Probe, once released, cannot
talk to the Galileo Orbiter until it begins to enter Jupiter's atmosphere,
we wanted to be very sure that everything was working fine before we let
it go. Therefore, the Probe Engineering Team asked that we first turn
on the Probe using an electrical cable from the Orbiter to make sure everything
was working correctly. Then we had to switch the Probe to its own internal
power before cutting this power cable (just like a baby being born gets
separated from its mother!).

A second challenge was that the computer equipment used by the Probe
Engineering Team is not the same as used by the rest of JPL. Therefore,
we needed to record the Probe information on special data tapes. For each
of these two key events - turning on the Probe and switching it to its
own internal power, we commanded the Galileo spacecraft to issue two status
reports each time. This allowed us to be VERY sure that there were no
problems with the spacecraft reports.

As it turned out, this was a lengthy process, with data coming in from
the spacecraft as late as 10:30 p.m. and these tapes being delivered sometimes
at 2:30 am.

My job in all this was to identify where the data was going when it
arrived at the Deep Space Network and to negotiate between the Probe Engineering
Team and the Data Management team (the data tape folks) this rather unusual
set of delivery times. In the end, everything ran very smoothly and the
Probe Release on Wednesday July 12 went off perfectly!

We will be going through a similar process in December when the Probe
enters the atmosphere and starts sending back "real data". Even now I
am updating that same tape delivery schedule. Keep your fingers crossed!